Yet these "gimmicks" have helped TMobile generate increasing Operating Cash Flow each year for over the past 5 years.
And in case you don't know what Operating Cash Flow is....
http://www.investopedia.com/terms/o/operatingcashflow.asp
Operating Cash Flow (OCF) is a measure of the amount of cash generated by a company's normal business operations. Operating cash flow is important because it indicates whether a company is able to generate sufficient positive cash flow to maintain and grow its operations, or whether it may require external financing.
So yeah, I think TMobile is doing just fine offering these "gimmicks." I don't see where these "gimmicks" have negatively affected their business. In fact, they have helped the company and their subscriber base grow. 10 months ago (Aug 2015) they overtook Sprint as the #3 U.S. carrier.
And with an almost 38% gross profit margin (almost as high as Apple's, by the way), they can afford to offer more of these "gimmicks" down the road if they wanted to.